A variety of instruments and electronic measuring devices rely on the measurement of resistance or changes in resistance of conductive elements disposed in a flow environment. Such instruments may include sensors that allow the determination of fluid flow rates based on heat transfer to or from a fluid stream. These may include hot-wire sensors and thin film sensors such as those described in U.S. Pat. No. 5,218,863 (“'863 patent”), U.S. Pat. No. 5,074,147 (“'147 patent”), U.S. Pat. No. 6,963,810 (“'810 patent”), U.S. Pat. No. 6,826,493 (“'493 patent”), and U.S. Pat. No. 7,132,838 (“'838 patent) al of which are incorporated herein by reference in their entireties.
Resistance-based instruments typically comprise a detector filament that may be positioned in a measurement configuration/environment. The detector element forms a part of a sensor circuit that may be used to identify changes in the resistance of the element. In anemometer devices, the detector element is a filament heated by electric current while immersed in a fluid flow stream. The passage of the fluid stream over the hot filament heats or cools the filament and consequently causes its electrical resistance to change.
In prior art flow sensor systems, the detector element was typically disposed in one arm of a Wheatstone bridge circuit and supplied with a source of electrical energy. A difference of potential, the value of which is mathematically relatable to the heat transfer from the fluid stream, appears across the measuring diagonal of the Wheatstone bridge. Bridge circuits, however, have several drawbacks that limit their use in many sensor applications. For example, in order for the circuit to provide meaningful data, the bridge must be balanced. Also, the bridge output sensitivity to resistance changes is inherently limited, thus necessitating large subsequent amplification. It is a well-known fact that such amplification will deteriorate the signal-to-noise ratio of the measured data. Finally, bridge circuits suffer greatly from electromagnetic and radio-frequency interference (EMI/RFI) especially with flow sensors exposed to the environment.
In prior art fluid flow measurement systems incorporating temperature-compensation (e.g., U.S. Pat. Nos. 4,807,151, 4,845,984, 4,566,310, 4,490,803, 5,063,352, 7,003,418), the bridge circuit is primary used. As a result, they all suffer from the issues previously mentioned, and cannot resolve these issues without significant compromises in sensitivity bandwidth or circuit complexity.